Prebake anodes for electrolytic production of aluminum and coating therefor



Feb- 22, 1966 H. sKANTzE: ETAI..

PREBAKE ANODES FOR ELECTROL'YTIC PRODUCTON OF ALUMINUM AND COATING' THEREFOR Flled MICh 2l 1961 Pif/5,41% AfA/005 STEM IN V EN TOR.S

PREBAKE ANGDES EUR ELECTRQLYTHC PRO- DUCHUN F ALUMNUM AND CUATING THEREFOR Hugo Sltantze, Stockholm, Sweden, and Christopher .lohn

English, Arvida, Quebec, Canada, assignors to Aluminium Laboratories Limited, Montreal, Quebec, Canada, a corporation of Canada Filed Mar. 2l, 1961, Ser. No. 97,280 9 Claims. (Cl. 204-67) This invention relates to the production of aluminum. More particularly, this invention relates to the electrolytic process, the so-called Hall or Heroult process, for the production of aluminum wherein current is passed through a molten electrolyte bath consisting essentially of cryolite containing alumina d-issolved therein. Upon passage of current through the molten electrolyte bath, elemental aluminum in molten form is deposited at the cathode and, being more dense than the electrolyte bath, sinks to the bottom of the Ibath of molten electrolyte from which it is recovered from time to time.

The above, generally described electrolytic process for the manufacture of `aluminum is carried out in electrolytic cells or pots which are relatively shallow containers made up of an outer steel shell, an intermediate lining of insulating, refractory material, such as alumina or insulating brick, and an inner carbon lining which is provided with suitable conductors and which serves as an electrode (cathode). The bath of molten electrolyte, cryolite or sodium fluoride-aluminum fluoride material containing alumina dissolved therein, usually in an amount of from about 2% by weight of the electrolyte bath to saturation with alumina, is in contact with the carbon lining or cathode of the cell. There is immersed in the electrolyte bath one or more anodes `also formed of carbon and containing a conductor imbedded therein. As curnent passes through the cell via the electrodes, anode and cathode, and the electrolyte bath, the resistance offered to .the flow of -current by the bath of molten electrolyte which is usually at a temperature in the range 900-1000" C. serves to maintain the electrolyte in a molten condition and at the above-mentioned operating temperature.

Elemental, molten aluminum is deposited at the cathode lining upon passage of current through the electrolyte bath and is removed from the cell from time to time. As the alumina content of the electrolyte bath becomes depleted additional alumina is added. Simultaneously wi-th the production of molten aluminum at the cathode, oxygen is produced at the anode and combines with the carbon of the anode with the resultant production of carbon dioxide and some car-bon monoxide. These gaseous materials together with a small amount of volatilized, normally solid materials from the bath are separately drawn off and treated for the recovery of any values, e.g. cryolite or lluorine-containing material, therefrom.

It is thus seen that in the electrolyt-ic process for the manufacture of aluminum the carbon anodes are consumed as aluminum is produced. It has been estimated that for every pound of aluminum produced about 0.5-0.6 pound of carbon electrodes, principally carbon anode material, is consumed. It is evident, therefore, that a substantial portion of the cost involved in fthe produc-tion of aluminum is due to anode consumption. Additionally, because the electrolyte cells are operated at a rather high temperature in the range 950-1000 C., more or less, the carbon anodes become Very hot and as a result are subject to oxidative `degradation and are to some extent actually consumed by burning on -contact with air or any oxygen-containing gas in contact therewith.

Heretofore, various techniques have been suggested and States Patent employed to increase the life of the carbon anodes, especially .to protect the carbon anodes against combustion or oxidative degradation in contact with air or other oxygencontaining gas. These techniques have included coating or applying :to the surface of the carbon anode materials such as sodium silicate (Water glass), boric acid, aluminum paint and aluminum itself. For the most part, these techniques have not proved to be satisfactory. In the case of cladding or coating the anode with a sheath or mantle of aluminum, as the anode is consumed the aluminum melts into the electrolyte bath and, in effect, the aluminum employed to coat the carbon anode is recirculated Within the cell creating an undesirable circulating load of metal in the plant. Additionally, a considerable amount of aluminum, which otherwise could be sold, is withheld as anode coating material.

Ac-cordingly, -it is an object of this invention to provide an improved prebake carbon anode for use in connection with the production of aluminum.

Another object of this invention is to provide an improved process for the manufacture of aluminum involving an operation wherein current is passed through an electrolyte bath containing alumina dissolved in cryolite or similar sodium fluoride-aluminum fluoride material as solvent for the alumina.

Another object of this invent-ion is to provide a composition of matter suitable as an anode coating composition.

Still another object of this invention is to provide a method for protecting carbon anodes employed in the electrolytic production of aluminum from attack by oxygen.

Ye-t another object of this invention is to provide a method for the manufacture of carbon anodes suitable for employment in the electrolytic production of aluminum.

Still another object of this invention is to provide a carbon anode useful in the electrolytic production of aluminum and having an increased operating life.

Yet another object of this invention is to provide a method whereby the consumption of carbon anode material per unit Weight of electrolytically produced aluminum is reduced.

How these and other objects of this invention are achieved will become apparent in the light of the accompanying disclosure and with reference to the accompanying drawings wherein:

FIG. l is a partial cross sectional view in perspective of a carbon anode prepared in accordance with this invention; and wherein FIG. 2 is a partial cross sectional view of an electrolytic cell or pot suitable for use in the electrolytic refining of aluminum and employing carbon anodes prepared in accordance with this invention.

In accordance with this invention an improved anode suitable for use in the electrolytic production of aluminum is provided 'by coating the anode with a film vor coating of a composition comprising synthetic cryolite (Na3AlF6 or 3NaF.AlF3) plus an excess of aluminum fluoride in the range l5-.35% by weight (hereinafter sometimes referred to as synthetic cryolite material or sodium fluoride-aluminum fluoride material), or similar sodium fluoride-aluminum fluoride material having a Weight ratio of sodium fluoride to aluminum fluoride in the range from about 0.6 to about 1.2, said composition having a melting p-oint in the range of about 700 `C. to about 1000 C. In accordance with one embodiment of this invention there is advantageously incorporated in the coating composition a minor amount by weight of alumina. Accordingly, the anode coating composition of this invention consists essentially of a major amount, above 50% by weight, usually in the range 70-95% by weight, of synthetic cryolite, together with an excess of aluminum uoride, or similar sodium fluoride-aluminum fluoride material having a weight ratio of sodium fluoride to aluminum fluoride in the range from about 0.6 to about 1.2, advantageously along with a minor amount by weight of alumina, such as an amount in the range 3-8% by weight alumina. Further, desirably, when only synthetic cryolite plus excess aluminum fluoride and alumina and calcium fluoride (usually present as an impurity below about 2.0% by weight) comprise the constituent materials of the coating composition the weight ratio of sodium fluoride to aluminum fluoride in the above-described synthetic cryolite material is about 0.8 and the amount of alumina present is about 5% by weight and the amount of calcium Fluoride is about 0.5% by weight.

In acordance with another embodiment of this invention, in addition to cryolite plus excess aluminum fluoride or similar sodium fluoride-aluminum uoride material and alumina, there may also advantageously be included in the anode coating composition a minor amount of salt, sodium chloride, such as an amount of sodium chloride in the range -25% by weight. When sodium chloride is employed as a constituent in the coating composition the synthetic cryolite plus excess aluminum or sodium fluoride-aluminum fluoride material present therewith desirably has a weight ratio of sodium fluoride to aluminum fluoride of about 1.1. A particularly suitable coating composition prepared in accordance with this embodiment of the invention has the composition about 19-20% 'by weight sodium chloride, about 4% by weight alumina, about 0.5% by weight calcium fluoride, the remainder being synthetic cryolite plus excess aluminum fluoride or sodium fluoride-aluminum fluoride material having a weight `ratio of soduim uoride to aluminum fluoride of about 1.1.

The coating composition prepared in accordance with this invention has a melting7 point below the operating temperature of the electrolytic cell with which it is employed in combination as an anode coating composition. Accordingly, the coating composition has a melting point below about 10G0 C., more or less. Desirably, the coating composition has a melting point in the .range 700 C. to about 950 C., more or less, depending upon the specic composition of the electrolyte employed in the electrolytic cell for the manufacture of aluminum and the cell operating temperature. However, the melting point of the anode coating composition should be higher than the actual surface temperature of the carbon anode employed in the electrolytic production operation, i.e. the melting point of the coating composition should be above about 700 C.

In the preparation of the coating composition of this invention a solid admixture of the various ingredients making up the coating can be heated together to effect melting of all the ingredients into a single molten mass; or the major constituents, the synthetic cryolite plus excess aluminum fluoride can be melted and then the various other ingredients such as alumina, calcium fluoride and sodium chloride admixed therewith and melted therein.

In the preparation of a coated carbon anode in accordance with this invention, after a coherent, integral, self-supporting, solid and substantially non-porous or impermeable mass of carbon has been formed into the desired electrode shape and a metallic conductor such as a bar of copper, iron, aluminum ora suitable electrically conductive metal imbedded therein, the resulting formed carbon anode is dipped into a pool of molten coating composition. Before the carbon anode is dipped into the pool of coating composition it is desirable that the carbon `anode be preheated to a uniform temperature in the range about 400-600 C. The pool of molten coating material during the dipping and coating operation should be maintained at a temperature of about 50 degrees centrigrade above the melting point of the coating composition. More than one immersion of the carbon anode into the pool of molten coating composition may be required to effect a suitable coating on the carbon anode. It has been found that a coating having `a thickness in the range from about 1/16 to about 1/s inch thick yields satisfactory results.

Referring now to FIG. 1 of the drawings there is illustrated therein a carbon anode prepared in accordance with this invention. As illustrated a conductor 11 is shown imbedded in the body of the carbon anode generally indicated by reference numeral 12, the carbon anode comprising a suitably formed, substantially solid, non-porous, impermeable mass of carbon 14 provided with a substantially liquid impermeable coating 15 having a composition in accordance with this invention.

Referring now to FIG. 2 of the drawing which schematically shows an arangement of carbon anodes prepared in accordance with this invention in an electrolytic cell or pot for the electrolytic refining of aluminum. As illustrated in FIG. 2 the electrolytic cell comprises an outer stell shell V20, an intermediate insulating, refractory lining 21 of suitable material, such `as alumina, and an inner coating or lining 22 of carbon. There is imbeded in carbon lining 22 conductor bars 24 made of suitable electrically conducting metal such as iron, copper, alumminum or an electrically conducting metal alloy. A pool of molten aluminum is indicated by reference numeral 25 lying at the bottom of molten electrolyte bath 26 which is covered by :a crust 27 of solid electrolyte. Carbon anodes 28 prepared in accordance with this invention are immersed part of the way into molten electrolyte bath 26, suspended on electrically conductive bars 29. Bars 29, `in turn, are operatively connected to and supported on bus bar or conductor 30 whereby upon application of a suitable voltage differential lbetween bus bar 30 and conductors 21 a current is caused to flow through anodes 28, electrolyte bath 26 and carbon lining 22 and conductors 24, the carbon `lining 22 serving as the cathode upon which molten aluminum is deposited.

One important feature of the coating composition prepared in accordance with this invention is the ability of the coating composition in the molten state to wet the surfaces of the carbon `anode as the carbon I.anode is immersed in the molten coating composition during the dipping or coating operation. Any surface cracks imperfections or incipient surface cracks on the surface of the anode are penetrated by the molten coating composition :and are effectively sealed off by the coating composition iwhen the coating sol'idifies on the anode to form a protective, fluid impermeable sheath thereon. In addition to sealing any cracks or lling surface imperfections on the carbon anode the coating composition tends to strengthen the carbon body of the anode and to protect the same during handling. To this end it is desirable that the coating composition contain not more than about 1% by weight calcium, usually in the form of calcium fluoride, since it has been found that calcium uoride adversely affects the wettability of the coating composition with respect to carbon and also makes the resulting coating more brittle.

The following examples are illustrative of the practice of this invention:

EXAMPLE NO. 1

A number of carbon anodes coated with a coating composition in accordance with this invention were placed in an operating cell or pot for the electrolytic production of aluminum The coated anodes were placed on the standard 21 shift changing schedule and alternated with another number of regular, unprotected, uncoated carbon anodes for comparison. While the regular, uncoated lanodes eroded noticeably, the coated carbon anodes showed no visible effects due to air oxidation during the first 7 to 12 shifts. Afterwards it was observed, however,

that the side coating peeled off because the coating fused to the crust of electroyte on top of the molten bath of electrolyte in the cell. The tops of the coated anodes, however, were still protected. Upon completion of the test runs wherein the coated and uncoated carbon anodes were employed for substantially the same length of time, the anode butts were measured and it was found that the coated anode butts were twice as large as the uncoated anode butts. The results of the butt measurements are set forth 1n accompanying 'Ila'ble I:

Table l Overall Overall Overall Height Length Width Coated Anodes:

No. 1 8% 14% 12 N0. 2 7% 14% 12 No 3--- 6% 14 12% No. 4 6% 15% 12 No. 5 8 14% 13 N0. 6.- 5 15 13 N0. 7 7 14% 13 No. 8 7% 15 12% N0. 9 7 14% 13% No. 10- 8 12% No. 11 7 13% 13 No. 12 7% 14% 12% N0. 13--- 5 15 12% No. 14 6% 16 15 Average 7 14% 12% Regular Uncoated Anodes:

No. 1 3 11 9 2 11 11 3% 11 l0 5 12% 11 5 11 10 5% 10 10 5 9% 9 5 10 9% 5 9% 10 5% 10% 10 Average 4% 10% 9% NorE.-Original anode size 13 x 17" x 13 high.

EXAMPLE NO. 2

Two anodes X 30 x 24" high and two anodes 20" x 35 x 24 high ywere coated with an anode coating composition consisting essentially of synthetic cryolite plus an excess of aluminum fluoride, the resulting sodium fluoride-aluminum fluoride material having a sodium fluoride to aluminum fluoride weight ratio of 1.1, 19% by weight sodium chloride, 4% by weight alumina and 0.5% by weight calcium fluoride. These anodes were tested in experimental pots used for the electrolytic pr-oduction of aluminum which otherwise used metallic aluminum coated anodes of substantially the same dimensions. lt Was observed that the coating composition of this invention gave the anodes as good protection against air oxidation as metallic aluminum. It was apparent that the coated anodes prepared in accordance with this invention were adequately protected and that the coating thereon was an effective and more economical and practical substitute for a metallic aluminum coating.

As will be apparent to those skilled in the art in the light of the accompanying disclosure, many modifications, changes and substitutions are possible in the practice of this invention without departing from the spirit or scope thereof.

We claim:

1. An article of manufacture suitable for use as an electrode in the electrolytic process for the manufacture of aluminum comprising a mass of carbon, a metallic electrical conductor imbedded in said mass of carbon and a coating provided on the outside of said mass of carbon, said coating, consisting essentially of a major amount by `weight `of synthetic cryolite (AlF3-3NaF) having an excess of aluminum fluoride (AlF3) therein to yield a weight ratio of sodium fluoride (NaF) to aluminum fluoride in the range from about 0.6 to about 1.2, a minor amount by weight of alumina (A1203), and less than about 1% by weight calcium fluoride said coating being impermeable and having a melting point in the range from about 700 C. to about 1000 C.

2. An article of manufacture suitable for use as an electrode in the electrolytic process for the manufacture of aluminum comprising a mass of carbon, a metallic electrical conductor imbedded in said mass of carbon and a coating provided on the outside of said mass of carbon, said coating consisting essentially of a major amount by weight of synthetic cryolite (AlF3-3NaF) having an excess of aluminum fluoride (AlF3) therein to yield a weight ratio of sodium fluoride (NaF) to aluminum lluoride in the range from about 0.6 to about 1.2 and about 5% by Aweight alumina (A1203) and about 0.5% by weight calcium iluoride (CaFZ), said coating being impermeable and having a melting point in the range from about 700 C. to about 1000 C.

3. An article of manufacture suitable for use as an electrode in the electrolytic process for the manufacture of aluminum comprising a mass of carbon, a metallic electrical conductor imbedded in said mass of carbon and a coating provided on the outside of said mass of carbon, said coating consisting essentially of a major amount by Weight of synthetic cryolite (AlF33NaF) having an excess of aluminum fluoride (AlF3) to yield a weight ratio of sodium fluoride (NaF) to aluminum fluoride therein in the range from about 01.6 to about 1.2 and minor amounts by weight alumina (A1203) and calcium fluoride (CaFg), the Cal-i2 content being not greater than about 1.0% by weight of said coating, said coating being impermeable and having a melting point in the range from about 700 C. to about 1000 C.

4. An article of manufacture suitable for use as an electrode in the electrolytic process for the manufacture of aluminum comprising a mass of carbon, a metallic electrical conductor imbedded in said mass of carbon and a coating provided on the outside of said mass of carbon, said coating consisting essentially of a major amount by weight of synthetic cryolite (AlF3-3NaF) having an excess of aluminum fluoride (AlFs) therein to yield a weight ratio of sodium lluoride (NaF) to aluminum fluoride in the range from about 0.6 to about 1.2, about 20% by weight sodium chloride (NaCl), about 5% by weight alumina and about 0.5% by Iweight calcium fluoride, said coating being impermeable and having a melting point in the range from about 700 C. to about 1000 C.

5. A method for the manufacture of an electrode sultable for use in the electrolytic process for the manufacture of aluminum comprising, forming a coherent, integral solid mass of carbon and depositing on the surface of said mass of carbon as an impermeable coating a composition consisting essentially of a major amount by Weight of synthetic cryolite (AlF3-3NaF) having an excess of aluminum lluoride (AlF3) therein to yield a weight ratio of sodium fluoride (NaF) to aluminum iluoride in the range from about 0.6 to about 1.2 a minor amount by weight of alumina (A1203), and not more than about 1% by weight `calcium fluoride, said composition having a melting point in the range from about 700 C. to about 1000 C.

6. A method in accordance with claim 5 wherein said mass of carbon is preheated to a uniform temperature in the range from about 400 C. to about 600 C. prior to depositing said coating `on the surface thereof.

7. A method in accordance with claim 6 wherein said composition is preheated and maintained at a temperature of about 50 C. above the melting temperature thereof during said depositing of said coating on the surface of said mass of carbon.

8. In the electrolyitc process for the manufacture of aluminum wherein an anode is immersed in an electrolyte bath comprising molten cryolite (AlF3-3NaF) containing alumina (A1203) dissolved therein and wherein current is passed through said bath between an anode and a cathode in contact with said bath with the resultant production of molten, elemental aluminum at the cathode, the improvement which comprises employing as the anode in said process a mass of coherent, substantially solid, non-porous carbon, a metallic electrical conductor imbedded in said mass of carbon and a coating provided on the outside of said mass of carbon, said coating consisting essentially of a major amount by Weight of synthetic cryolite (AlF3-3NaF) having an excess of aluminum fluoride (AlF3) therein to yield a Weight ratio of sodium fluoride (NaF) to aluminum fluoride therein in the range from about 0.6 to about 1.2, a minor amount by weight of alumina (A1203), and not more than about 1% weight calcium fluoride, said coating being impermeable and having a melting point in the range from about 700 C. to about 1000 C.

9. An article of manufacutre suitable for use as an electrode in the electrolytic process for the manufacture of aluminum comprising a mass of carbon, a metallic electrical conductor imbedded in said mass of carbon and a coating provided on the, outside of said mass of carbon, said coating consisting essentially of a major amount by weight of synthetic cryolite (AlF33NaF) plus an excess of aluminum fluoride (AlF3) to yield an aluminum iluoride-sodium fluoride material having a weight ratio of sodium fluoride (NaF) to aluminum fluoride therein in the range from about 0.6 to about 1.2 and a minor amount not more than about 1.0% by Weight calcium uoride, said coating being impermeable and having a `melting point in the range from about 700 C. to about 1000" C.

References Cited by the Examiner UNITED STATES PATENTS 1,083,691 1/1914 Mcllhiney 204-67 1,531,528 3/1925 Tilson 204-67 2,915,443 12/1959 Wallace et al 204-67 3,016,340 1/1962 Hygen 204-67 3,060,115 10/1962 Haupin et al. 204-290 FOREIGN PATENTS 120,475 10/1945 Australia.

JOHN H. MACK, Primary Examiner. 

8. IN THE ELECTROLYTIC PROCESS FOR THE MANUFACTURE OF ALUMINUM WHEREIN AN ANODE IS IMMERSED IN AN ELECTROLYTE BATH COMPRISING MOLTEN CRYOLITE (ALF3.3NAF) CONTAINING ALUMINA (AL2O3) DISSOLVED THEREIN AND WHEREIN CURRENT IS PASSED THROUGH SAID BATH BETWEEN AN ANODE AND A CATHODE IN CONTACT WITH SAID BATH WITH THE RESULTANT PRODUCTION OF MOLTEN, ELEMENTAL ALUMINUM AT THE CATHODE, THE IMPROVEMENT WHICH COMPRISES EMPLOYING AS THE ANODE IN SAID PROCESS A MASS OF COHERENT, SUBSTANTIALLY SOLID, NON-POROUS CARBON, A METALLIC ELECTRICAL CONDUCTOR IMBEDDED IN SAID MASS OF CARB ON AND A COATING PROVIDED ON THE OUTSIDE OF SAID MASS OF CARBON, SAID COATING CONSISTING ESSENTIALLY OF A MAJOR AMOUNT BY WEIGHT OF SYNTHETIC CRYOLITE (ALF3.3NAF) HAVING AN EXCESS OF ALUMINUM FLUORIDE (ALF3) THEREIN TO YIELD A WEIGHT RATIO OF SODIUM FLUORIDE (NAF) TO ALUMINUM FLUORIDE THEREIN IN THE RANGE FROM ABOUT 0.6 TO ABOUT 1.2, A MINOR AMOUNT BY WEIGHT OF ALUMINA (AL2O3) AND NOT MORE THAN ABOUT 1% WEIGHT CALCIUM FLUORIDE, SAID COATING BEING IMPERMEABLE AND HAVING A MELTING POINT IN THE RANGE FROM ABOUT 700*C. TO ABOUT 1000*C. 